Modern aircraft rely on efficient heat sink options for thermal management. The jet fuel that is supplied to the propulsion engines is often a convenient sink for excess thermal energy, and the energy is efficiently retained in the engine thermodynamic cycle. The presence of molecular oxygen or entrained air limits the ability of fuel to absorb heat beyond approximately 300° F. without undergoing deleterious thermal degradation. Thermal degradation often appears as solid materials which adhere to surfaces and degrades fuel system performance increase. Moreover, wetted surfaces comprised of metallic materials can further catalyze the reaction of oxygen with fuel and subsequent formation of carbonaceous, coke-like material.
It is possible to substantially reduce coke-based fuel degradation by removing oxygen from the fuel prior to increasing the fuel temperature beyond about 300° F. Several deoxygenation techniques have been developed. However, these often use equipment that is subject to fouling, which can lead to increased maintenance, and/or process steps that are difficult to control. The equipment used for fuel deoxygenation is also implemented separate from the aircraft engine. It would be preferable, therefore, to implement a fuel deoxygenation system as part of an aircraft engine fuel flow control system. It is generally known, however, that fuel deoxygenation systems typically perform better at operating pressures that are lower than some operating modes of an aircraft fuel flow control system. For example, the operating pressures of the fuel supply system may be higher during some non-cruise operations.
It would therefore be desirable to selectively reduce the discharge pressure of one or more fuel pumps within the deoxygenation system during some engine operational modes, while still meeting fuel supply system requirements in other operational modes. The present disclosure addresses at least this need.